Field of the Invention
The invention concerns a method for operating a magnetic resonance apparatus for checking a scanning protocol, described by recording parameters, that is to be implemented. The invention also concerns a magnetic resonance apparatus and an electronically readable data storage medium that implement such a method.
Description of the Prior Art
Magnetic resonance imaging (MRI) is a widely known imaging modality and is frequently used when examining medical issues. Magnetic resonance imaging uses a strong magnetic field, for example of 1.5 tesla or 3 tesla, so nuclear spins in a subject orient themselves along this basic magnetic field (B0). These uniformly aligned nuclear spins are excited by radio-frequency pulses and the decay of this excitation can be measured. To be able to allocate measured magnetic resonance signals to a location, magnetic gradient fields are used that are generated by appropriate gradient coils of a magnetic resonance scanner. The gradient coil arrangement is conventionally arranged inside an opening or tunnel through the basic field magnet in the scanner, into which the patient is moved in order to implement a patient scan.
During the scan, the gradient fields must be switched and changed extremely quickly using what are known as gradient pulses. Different frequencies can be generated by the physical gradient axes of the gradient coils of the gradient coil arrangement as a function of the scanning sequence that is used, recording parameters, and forms of the gradient pulses in the time domain. The spectral distribution of the corresponding frequencies has no effect on the hardware of the magnetic resonance scanner. Particular frequency bands can excite acoustic resonances and therefore lead to increased noise in the magnetic resonance scanner. Furthermore, strong gradients in particular frequency bands can also lead to increased interaction between the gradient coil arrangement and the basic field magnet. These excitations can lead to increased heating inside the basic field magnet, which can cause a coolant, in particular helium, used for cooling the basic field magnet to evaporate, or greater cooling can be needed in order to avoid a quench. Frequencies of this kind, which can lead to undesirable, interfering effects in the magnetic resonance scanner, will be called interference frequencies hereinafter, and these can also lie in interference frequency bands.
It would be possible for interference frequencies of this kind to be prohibited outright, and be taken into account in the design of scanning protocols or in the design of individual scanning sequences such that they do not occur. Such an approach, however, would severely limit the freedom of definition of scanning protocols and scanning sequences. Another approach would be to provide a passive frequency monitor that can be easily implemented as hardware in the magnetic resonance scanner, which monitors the excited frequencies during an ongoing magnetic resonance scan and stops the scan as soon as an excessive excitation of an interference frequency occurs. However, this does not prevent frequencies and scanning protocols, which lead to frequency errors, from being adjusted and started by a user. The scanning time of the magnetic resonance scan before stoppage is lost. If the frequency limitations for a particular magnetic resonance scanner tend to be minimal, then this would be a practicable solution, but this does not apply to many magnetic resonance scanners, so a passive frequency monitor of this kind severely impedes the workflow of the user and significantly increases total scanning times.
A further problem for the user is that the behavior of the magnetic resonance scanner is not predictable when a passive frequency monitor is used or with basic exclusion of the interference frequencies. Even for developers of scanning protocols and scanning sequences it is nearly impossible to estimate the frequency distribution, particularly due to the gradient pulses, during a magnetic resonance scan. The end result is therefore a highly non-linear, unpredictable, and unanalytical behavior of recording parameters with regard to their availability or suitability for a particular sequence.
A further problem can result from the fact that the physical gradient axes specified by the gradient coils of the gradient coil arrangement are not used in all scanning protocols and scanning sequences, wherein the latter conventionally form parts of a scanning protocol or an overall magnetic resonance sequence. Instead, gradient directions oriented in some other way may be used under some circumstances for slice selection gradients, phase-coding gradients and/or readout gradients. In other words, a gradient coil arrangement conventionally has three gradient coils, which can generate gradient fields in three physical gradient directions (conventionally x, y, z) that are perpendicular to each other. By appropriate control of the gradient coils and overlaying of the generated gradient fields, however, it is also possible to use different gradient directions compared to the physical gradient directions. These are conventionally called logical gradient directions or gradient axes. Within the context of the present invention this means that it is easily possible for scanning protocols to be operable in some orientations of the gradient directions without excessive excitations in the case of interference frequencies, while problems can occur with a rotation of these logical gradient directions.
With respect to other limitations of conventional magnetic resonance apparatuses, it has been proposed in subsequently published DE 10 2016 200 549.9 to supply at least one limitation by a limitation supply unit and to supply a number of parameters of the sequence by a parameter supply unit, wherein a pre-set parameter value is associated with at least one, ideally each, of the multiple parameters. Also in this procedure, one of the multiple parameters is selected by a selection unit, and a simulation unit determines at least one sequence progression using at least one of the pre-set parameter values. An evaluation unit then determines an admissible parameter value range of the selected parameter using the at least one sequence progression and using the at least one limitation, whereby a new parameter value is defined within the admissible parameter value range by a defining unit.
The device limitations, for example, can include a limitation dictated by the design of the magnetic resonance scanner with which the magnetic resonance tomography is to be carried out. The aforementioned subsequently published document refers, in particular, to a maximum gradient amplitude and/or a maximum slew rate, which can be generated by a gradient coil arrangement of the magnetic resonance scanner. An application limitation can also be used, namely a limitation that is dictated by an effect of the magnetic resonance scanner and/or an examination object, in particular a human or animal patient, due to implementation of a magnetic resonance sequence. It is possible, for example, that an application of excessive gradient amplitudes will affect physiological limits of the patient, such as peripheral nerves of the patient being excessively stimulated. Limitations can include a safety buffer to prevent that from happening. Frequency effects are not discussed in the subsequently published DE 10 2016 200 549.9.